This invention relates to an improved process for dewaxing hydrocarbon oils, particularly petroleum oils, most particularly lube oils wherein said waxy oil is introduced at a temperature above its cloud point into a first chilling zone divided into a plurality of stages, passing said waxy oil from stage-to-stage of said chilling zone, introducing a cold dewaxing solvent into at least a portion of said stages, whereby a solvent-waxy oil mixture is formed, maintaining a high degree of agitation in at least a portion of the stages containing solvent and waxy oil, thereby effecting substantially instantaneous mixing of said solvent and said waxy oil while cooling said solvent-waxy oil mixture, preferably at a rate of from 1.degree. to 8.degree. F. per minute, as it progresses through said first chilling zone to a temperature greater than the temperature at which the wax is separated from the oil, i.e., the separation temperature, but at least about 30.degree. F. above but less than about 40.degree. to 45.degree. F. above said separation temperature, whereby a substantial portion of the wax is precipitated from said waxy oil under conditions of said high degree of agitation and forming a solvent-oil mixture containing precipitated wax, withdrawing said mixture containing precipitated wax from said first chilling zone, and cooling the same to the separation temperature in a second chilling zone comprising scraped surface chillers, thereby precipitating a further portion of said wax from said waxy oil and separating said wax from the oil-solvent in solid-liquid separation means, the improvement comprising operating the scraped surface chillers of the second chilling zone at a chilling temperature range of at least 30.degree. F. and at a rotating element speed of only from 5 to 20%, preferably 8 to 14% of the original design operating speed. Scraped surface chillers typically operate at rotating element speeds of from 14 to 30 revolutions per minute. This approximately 10 fold decrease in scraper element spped results in obtaining wax filtration rates improved on the order of 10 to preferably 15 to 20%, while heat transfer coefficients are either uneffected or reduced by only about 15%. This loss of heat transfer efficiency is more than compensated by the improved wax filtration rates obtained.
It is known in the prior art to dewax hydrocarbon oilstocks by cooling an oil-solvent solution in scraped surface heat exchangers before separating the crystallized wax from the oil by physical means. In U.S. Pat. No. 3,775,288 it is taught that scraped surface heat exchangers can be used as a secondary cooling zone for the dewaxing of hydrocarbon oils following a primary cooling zone in which oil is cooled by contacting said oil with a cold solvent at a plurality of points along a vertical tower while maintaining a zone of intense agitation in at least a portion of the points of solvent injection, such that substantially instaneous mixing occurs at each point, i.e., within a second or less. This first cooling zone has become known as DILCHILL, a registered service mark of Exxon Research and Engineering Company. In the standard DILCHILL operation, oil is cooled by the injection of a chosen dewaxing solvent along the various stages of the DILCHILL tower with intense agitation from the cloud point to a temperature about 40.degree.-45.degree. F. above the separation temperature of the wax-in-oil typically followed by additional cooling in scraped surface chillers to the separation temperature.
It has been discovered that the above process is improved by reducing the speed of the rotating elements in the scraped surface chiller to a speed of from about 5 to 20%, preferably 8-14% of the original design operation speed of the scraped surface chiller. The chilling temperature range of the scraped surface chiller is at least 30.degree. F. Operation at this reduced speed across the recited temperature range of chilling improves wax filtering rates by about 10 to 20%, preferably 15 to 20%, while not adversely effecting the heat transfer performance of the chillers. This approximately 10 fold decrease in scraper element speed resulting in improved wax separability (as by filtration) is accompanied by no more than a 15% loss of heat transfer efficiency. This loss of heat transfer efficiency is more than made up by the improved wax separation i.e., filtration, etc. rates obtained.
Any hydrocarbon oilstock, petroleum oilstock, distillate fraction or lube oil fraction may be dewaxed by the process of this invention. In general, these stocks will have a boiling range within the broad range of about 500.degree. F. to about 1300.degree. F. The preferred oil stocks are the lubricating oil and specialty oil fractions boiling in the range of 550.degree. F. to 1200.degree. F. These fractions may come from any source such as paraffinic crudes obtained from Aramco, Kuwait, the Pan Handle, North Louisiane, Western Canada, etc. The hydrocarbon oil stock may also be obtained from any of the synthetic crude processes now practiced or envisioned for the future such as coal liquefaction, tar sands extraction, shale oil recovery, etc.
Any low viscosity solvent for oil may be used in the process of this invention, representative of such solvents are the ketones having 3 to 6 carbon atoms such as acetone, methyleethylketone (MEK), and methylisobutylketone (MIBK) and the low molecular weight hydrocarbons such as ethane, propane, butane, propylene and the like, as well as the mixtures of the foregoing ketones and mixtures of the aforesaid ketones with C.sub.6 to C.sub.10 aromatic compounds such as benzene and toluene. In addition, halogenated low molecular weight hydrocarbons such as dichloromethane and dichloroethane and mixtures thereof may be used as solvents. Specific examples of suitable solvent mixtures are methylethylketone and methylisobutylketone; methylethylketone and toluene; acetone and toluene; acetone and propylene; benzene and toluene; dichloromethane and dichloroethane. The preferred solvents are the ketones. A particularly preferred solvent mixture is a mixture of methylethylketone and methylisobutylketone or a mixture of acetone and propylene. Another preferred solvent mixture is methylethylketone and toluene.
General operating conditions of the instant invention are recited and presented in detail in U.S. Pat. Nos. 3,775,288 and 3,773,650, both of which are incorporated by reference. The instant application is an improvement over both of these patents by demonstrating improved dewaxing and wax filterability by reducing the speed of the scraper element in the scraped surface chilling units which follow the DILCHILL process tower.
Scraped surface chillers such as those used in combination with DILCHILL towers described in U.S. Pat. Nos. 3,773,650 and 3,775,288 generally operate at rotating element speeds from 14 to 30 revolutions per minute. This rotation is in response to the need to remove wax from the chiller walls since build-up of wax on the cooling surfaces results in a substantial decrease in the chilling efficiency of the units. The build-up of wax on the chilling surfaces and internals also has the effect of effectively blocking the flow paths of the precipitated wax/oil solvent slurry increasing the pressure drop through the unit. It has now been surprisingly discovered that scraped surface chillers can be run efficiently at approximately a 10 fold decrease in rotating element scraper speed, i.e., at speeds of from 1.5 to 6 RPM, preferably 1.5 to 2.4 RPM and that such a speed reduction does not hamper the heat exchange ability of the chiller nor result in an unacceptable pressure drop across the chiller. The scraped surface chillers are operated so as to have a chilling temperature range of at least 30.degree. F. In fact, and surprisingly, it has been found that the wax coming from such a unit wherein the rotating element is operating at the reduced speed exhibits a surprisingly increase/improvement in separability (i.e., filterability) yielding a wax cake which does not clog the filter clothes or filtering means typically employed in a solid-liquid separating means. The frequency of wax removal from the wall is sufficient to maintain adequate heat transfer rates, but significantly reduces the addition of "wall crystals" to the slurry which are responsible for reducing filtration performance.